Guest Editorial/

Geophysics Is Not a Silver Bullet, but Worth a Shot by Kamini Singha

Hydrogeologists have long been interested in using the change in the geophysical properties is sometimes geophysical measurements to answer questions about the closely related to a hydrologic process. Scientists have Earth. Originally used to find ore bodies or oil and gas, successfully used to map the movement of geophysics’ sensitivity to water was originally considered water or tracers in , the , and a nuisance. While looking for metallic ore using electrical- in groundwater-surface water exchange. We continue to induced polarization, for instance, early geophysicists push the envelope by imaging root-water uptake, which found that groundwater controlled most of the measured can help us parameterize rooting depths for climate signal. Others later capitalized on this “nuisance” by using models. We can also map how microorganisms influence geophysical methods to explore for water directly, or to biogeochemical processes related to bioremediation. We attempt to estimate hydro(geo)logic properties. Unfortu- can image potential water stores in the subsurface. nately, no universally reliable relations between measured We are also scaling up; I believe airborne electro- geophysical properties, such as seismic velocity, and magnetics (AEM) platforms will become a game changer hydrologic properties of interest, like a concentration flux, for : Just as Light Detection and Ranging exist in the field. For instance, both positive and negative (LiDAR) has changed the way we think about surface correlations were found between hydraulic conductivity morphology, AEM can do the same for characterizing and electrical conductivity. So, early hydrogeophysics subsurface variability. research capitalized on imaging spatial differences in Geophysical methods are not only being commonly geophysical properties to map potential hydrogeologic used for hydro(geo)logic research, but they also have facies as much as the more difficult problem of attempting become practical tools for consultants. Valuable decision- to estimate hydrologic properties directly. support tools now help hydrogeologists find appropriate While Doll et al. (2012) note that the field of geophysical methods, for instance, for fractured-rock near-surface geophysics was booming in the 1980s, systems (Day-Lewis et al., 2016). Ongoing innovations environmental applications of geophysics were still only will continue to establish geophysical tools as part of a about 0.1% of reported geophysical activity in 1990. From hydrogeologist’s toolbox for the decades to come. While early years, some thought geophysics could be made to geophysics will continue to have utility in mapping “see” whatever the user wanted (e.g., Blau 1936), such changes in subsurface properties, we have not yet suc- that the methods were in “ill repute” within hydrogeology. ceeded in directly and reliably measuring properties like Fortunately, opinions are changing. hydraulic conductivity in the field. I am not convinced The field of hydrogeophysics really came into its own we will ever be able to robustly estimate these properties in the early to mid 2000s, in part by addressing problems at all field sites, but if someone builds a geophysical of interest to the hydrogeology community. To me, this instrument that can non-invasively measure hydraulic evolution was about asking the right questions with geo- conductivity directly at any site, I promise to buy two! physics, and using geophysics to complement traditional Editor’s Note: Prof. Kamini Singha is the Darcy methods and fill in gaps, rather than using it to substitute Lecturer for 2017. for hydrologic measurements. I’d always prefer a single borehole with a pressure transducer to trying to find the References water table with a geophysical method. That said, there is Blau, L.W. 1936. Black magic in geophysical prospecting. no better way to see between the sparse “hard” data than Geophysics 1, no. 1: 1–8. Day-Lewis, F.D., C.D. Johnson, L.D. Slater, J.L. Robinson, with geophysics. Geophysics can also map time-lapse J.H. Williams, C.L. Boyden, D.D. Werkema, and J.W. changes over broad spatial and temporal extents, and Lane. 2016. A fractured rock geophysical toolbox method selection tool. Groundwater 54, no. 3: 315–316. Geology and Geological Engineering and Hydrologic Science Doll, W.E., R.D. Miller, and J. Bradford. 2012. The emergence and Engineering, Colorado School of Mines, Golden, CO 80401; and future of near-surface geophysics. The Leading Edge [email protected] 31, no. 6: 684–692. © 2017, National Ground Water Association. doi: 10.1111/gwat.12495

NGWA.org Groundwater 1